1. Field of the Invention
This invention relates to the field of optical devices that manipulate optical energy of tightly controlled optical wavelength, particularly for use in communication applications. More particularly, the invention relates to (a) integrated optical components which, when coupled to optical energy generating means, interact with optical energy of a specified wavelength and which can be tuned or switched to other specified wavelengths by thermal means and (b) lasers which produce optical energy of a specified wavelength and which can be tuned or switched to other specified wavelengths by thermal means.
2. Description of the Related Art
Over the past several years, there has been ever increasing interest in tunable lasers for use in optical communication systems, in general, and for use in dense wavelength division multiplexing (DWDM) applications, in particular. DWDM allows high bandwidth use of existing optical fibers, but requires components that have a broad tunable range and a high spectral selectivity. Such components should be able to access a large number of wavelengths within the S-band (1490-1525 nanometers), the C-band (1528-1563 nanometers), and the L-band (1570-1605 nanometers), each different wavelength separated from adjacent wavelengths by a frequency separation of 100 MHz, 50 MHz, or perhaps even 25 MHz, according to the system implementation.
The tunable monolithic wavelength selective coupler was one of the first components used in optical communication. The wavelength selective coupler consisted of a pair of asynchronous waveguides in combination with a phase matching coarse grating for coupling optical energy between the waveguides. Tuning was achieved by either injecting current into or applying a reverse bias voltage to the coupler thereby changing the coupled wavelength. Electrical contacts on the outer surface of the coupler were provided for this purpose, namely, for applying a potential to or for injecting current into the coupler. For a more detailed discussion of a tunable monolithic wavelength selective coupler, please refer to U.S. Pat. No. 5,253,314 by Alferness et al. entitled “Tunable Optical Waveguide Coupler” which is hereby incorporated by reference.
The distributed Bragg reflector (DBR) laser was one of the first tunable lasers used in optical communication. The DBR laser consisted of a semiconductor amplifier medium, defining an active section, and an optical waveguide. The optical waveguide included a portion without a grating that defined a phase control section and a portion in which a single grating of typically constant pitch (Λ) was formed which constituted a distributed Bragg reflector or, more simply, the Bragg section, that reflected light at the Bragg wavelength λB. Wavelength tuning of such a DBR laser was performed by transferring heat into the phase control section, the Bragg section, or both. The optical waveguide was defined by an organic layer which constituted a core with another organic confinement layer disposed both above and below the core. Wavelength tuning of such a DBR laser was performed by either injecting current or transferring heat into the phase control section, the Bragg section, or both. Injecting minority carriers made it possible to vary the refractive index of the waveguide and thus control the Bragg wavelength λB by the equation λB=2neffΛ where Λ is the pitch of the grating and neff is the effective refractive index of the waveguide. Alternatively, a pair of heating resistance strips were disposed on opposite outer surfaces of the laser component at the phase control section, the Bragg section, or both. By independently controlling the voltages to the resistance strips, the temperature and hence the index of refraction of the organic layers that form the optical waveguide was controlled via the thermo-optical effect. Tuning by injecting current had the disadvantage of increasing optical loss and adding optical noise. Tuning by heating has the disadvantage of accelerating the aging of the device and thereby diminishing the useful life. Both options induce long-term drift in the Bragg wavelength thereby reducing reliability. For a more detailed discussion of a wavelength tunable DBR laser by heating, please refer to U.S. Pat. No. 5,732,102 by Bouadma entitled “Laser Component Having A Bragg Reflector of Organic Material, And Method of Making It” which is hereby incorporated by reference.
A grating assisted coupler with sampled rear reflector (GCSR) laser was another type of tunable laser that held great promise. The GCSR laser comprised of a semiconductor amplifier medium, a coupler section, a phase control section, and a reflector section. Electrodes were disposed on opposite outer surfaces of the GCSR laser for any or all of the coupler section, the phase control section, and the reflector section and wavelength tuning was performed by injecting current into any or all of the coupler section, the phase control section, or the reflector section. While providing a broad tuning range, wavelength tuning by injecting current causes considerable spectral line width broadening and a decrease in emitted power, both important criteria in DWDM applications. For a more detailed discussion of a wavelength tunable GCSR laser by injection current, please refer to U.S. Pat. No. Re. 36,710 by Baets et al. entitled “Integrated Tunable Optical Filter,” to a paper by Rigole et al. entitled “Access to 20 Evenly Distributed Wavelengths Over 100 nm Using Only a Single Current Tuning in a Four Electrode Monolithic Semiconductor Laser,” IEEE Photonics Technology Letters, Vol. 7, No. 11, Pages 1249-1251, November 1995, and to a paper by Rigole et al. entitled “State-of-the-art: Widely Tunable Lasers,” SPIE, Vol. 3001, Pages 382-393, 1997, which are all hereby incorporated by reference.